Although all cells in an organism have the same DNA, some cells express different portions of the DNA. The genes that are expressed directly affects-

the answer to your question is tooth decay.

b) the inside picture would look at a very high temperature in a very low ph

fumikage isn't really a bird, and tsuyu does have teeth. tsuyu has a transformation quirk which still allows her to be partially human. otherwise we'd have a large frog running around. fumikage has teeth because again, he isn't a bird. his head was a quirk mixture if i'm correct.